You push out all the air out of your lungs and hold your breath. You begin counting each beat of your heart.
As you reach ten, you think, "I'll make it to twenty and then take a breath." But twenty comes easily, so you decide on thirty instead: "Thirty will be easy; then I breathe in, for sure." However, by the time you count to twenty-three, discomfort sets in. You're pretty sure that your counting has sped up beyond your heart rate, and when you finally get to thirty and inhale- for some reason, being able to breathe again... doesn't feel like a relief as much you expected it to be.